Let's not forget the kindle syncs between devices...that is the point that sells Kindle for me, although I have several other readers loaded I always go to kindle. I can pick up mybphone and read from where I left off on the iPad, or pc or whatever
I was coming from the perspective of the average, not so tech savvy consumer who may not know how to use and format .ePub books and doesn't want to go through the process to learn how to format into .ePub just to read their books. All they really want, in my opinion, is just an easy way to read books across a number of different platforms - which I believe Kindle, in that respect even with their proprietary format, still does better than iBooks right now.
32GB black iPhone 4
16GB wifi-only iPad
The great thing about having both a Kindle and IPad is having the ebooks on both devices - I can read on one at home, one while on the train and it auto -updates where I am. Very cool. I hope Apple don't try and squeeze our access to Amazon etc out by charging for downloads to IPad as rumoured - it could ruin what is currently a lovely symbiosis!